How to make a single user copy of a group of nodes in matrial on python in Blender?
The copy function can be used on the node tree as well. The copy function returns the copied node group, so it can be used replace the original group with the single user copy:
original_group = bpy.context.objects['Object'].material_slots['Material'].material.node_tree.nodes['Baking_node'].node_tree
single_user_group = original_group.copy()
original_group = single_user_group 

Then just loop that over all your materials